Method and apparatus for image processing, including processing for reproducing black character areas of color and monochromatic images

ABSTRACT

In an image processor which converts color image data to image data of cyan, magenta, yellow and black necessary for forming an image, a black character edge is discriminated in R, G, B image data. The edge component in a black character area is deleted by narrowing on image data of cyan, magenta and yellow. Further, in a black character area, K data is replaced with maximum density data in the R, G, B data, and edge emphasis is performed on the substituted data. Then, reproducibility of a thin line portion in a character is improved largely.

This application is based on an application No. 10-69007/1998 filed inJapan, the content of which is incorporated hereinto by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image processor which receives colorimage data and converts them to image data used for image forming.

2. Description of Prior Art

Color image data of red, green and blue are converted to image data ofcyan, magenta, yellow and black for image forming. It is known todiscriminate black character areas in the input color image data and tothe black component is subjected to edge emphasis and the cyan, magentaand yellow components are subjected to erase edges.

In order to improve reproducibility of edges of black characters in acolor image, it is proposed to eliminate cyan, magenta and yellowcomponents at the character edges. That is, a minimum of the threecomponents is substituted to prevent partial whitening around the blackcharacters. An amount of edge correction on edge emphasis of blackcomponent is obtained by a spacial filter of lightness data forexpanding the characters in order to improve reproducibility of blackcharacters. On the other hand, in areas discriminated as blackcharacters, the amount of black paint for the black component is set to100% to increase the density of black characters.

However, even in the proposed processing, the density of blackcharacters is insufficient, and the characters are reproduced withdensities smaller a little than the original document. If the amount ofedge emphasis is increased further, a sufficient effect is observedthough the contrast at the edges of the black characters is enhanced.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an image processorwhich improves reproducibility of black characters.

In an image processor according to the invention which converts imagedata of red, green and blue to image data of cyan, magenta, yellow andblack, an area discriminator discriminates a black character area in theimage data of red, green and blue, and a substitution processorsubstitutes a maximum density in the image data of red, green and bluefor image data of black belonging to the black character areadiscriminated by the area discriminator. Then, an edge emphasisprocessor performs edge emphasis on the image data of black aftersubstitution by the substitution processor.

An advantage of the present invention is that the density level in blackcharacter areas becomes sufficient, especially for a thin line portionin a black character.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of the present invention willbecome clear from the following description taken in conjunction withthe preferred embodiments thereof with reference to the accompanyingdrawings, and in which:

FIG. 1 is a cross sectional view of a color digital copying machine;

FIG. 2 is a schematic illustration of a constitution of a laser opticalsystem;

FIGS. 3A and 3B are parts of a block diagram of an image processor;

FIGS. 4A and 4B are parts of a diagram on relationship between a systemconstitution of the copying machine and a print image controller block;

FIGS. 5A-5F show color shifts caused due to six types of factors;

FIG. 6 is a block diagram of the print image controller;

FIG. 7 is a diagram of an example of an image correction by a gradationlevel distribution;

FIG. 8 shows a print head controller;

FIGS. 9A, 9B and 9C are parts of a block diagram of the color corrector;

FIGS. 10A and 10B are parts of a block diagram of a regiondiscriminator;

FIG. 11 is a diagram of: a primary differential filter;

FIG. 12 is a diagram of a secondary differential filter;

FIG. 13 is a diagram on operation of a character/background boundaryidentifier;

FIG. 14 is a diagram on operation of a combination of two differentialfilters;

FIG. 15 is a diagram for illustrating operation of a character edgeprocessing;

FIG. 16 is a diagram on a concavity made due to an edge emphasis;

FIG. 17 is a graph of a chroma reference table;

FIG. 18 is a diagram for illustrating black judgment;

FIG. 19 is a diagram on image deterioration on a cross portion due to ageneration;

FIG. 20 is a diagram on an isolated dot condition decision fordiscriminating a dot;

FIG. 21 is a diagram on dot discrimination which is performed when acentral pixel is shifted;

FIGS. 22A and 22B are parts of a block diagram of a character edgereproducer;

FIG. 23 is a diagram of a Laplacian filter;

FIG. 24 is a diagram of a smoothing filter;

FIG. 25 is a diagram for showing an influence of LOG correction on theedge;

FIG. 26 is a diagram on the improvement of reproducibility of the edgeof a black thin line;

FIG. 27 is a diagram on the correction of the blurred color by a blackcharacter discrimination;

FIG. 28 is a block diagram of a gradation reproducer;

FIG. 29 is a block diagram of a 3-bit encoding;

FIGS. 30A and 30B are parts of a block diagram of a subscan drawingposition controller, and FIG. 30C is a diagram on the subscan drawingposition controller;

FIG. 31 is a block diagram of a main scan drawing position corrector;

FIGS. 32A and 32B are parts of a block diagram of an image distortioncorrector;

FIGS. 33A and 33B are parts of a block diagram of a subscan imagedistortion correction;

FIG. 34 is a block diagram of a gradation level decoder;

FIGS. 35A and 35B are parts of a block diagram of a main scan imagedistortion corrector;

FIG. 36 is a block diagram of an image distortion coefficient datagenerator;

FIG. 37 is a diagram on interface between the print image controller andthe print head controller;

FIG. 38 is a timing chart of data transmission from the print imagecontroller to the print head controller;

FIG. 39 is a diagram of a resist detecting pattern;

FIG. 40 is a diagram of subscan distortion correction;

FIG. 41 is a diagram of main scan distortion correction; and

FIGS. 42A and 42B are parts of a block diagram of a frame memory.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference charactersdesignate like or corresponding parts throughout the several views, FIG.1 shows a whole structure of a color digital copying machine. Thecopying machine comprises an automatic document feeder 100, an imagereader 200 and an image former (printer) 300. Typically, the imagereader 200 reads a document which is fed to an image read position bythe automatic document feeder 100, and then the read image data istransmitted to the image former 300, whereby an image can be formed(copy function). Also, an interface 207 enables the copying machine tobe connected to an external apparatus. Thus, the image data read by theimage reader 200 is outputted to the external apparatus (image readfunction), or conversely the image data received from the externalapparatus is sent to the image former 300, whereby the image can beformed (printer function).

Next, the automatic document feeder 100 is described. The automaticdocument feeder 100 feeds the document set on a tray 101 to the imageread position of the image reader 200. After the image read terminates,the document is delivered onto another tray 103.

The document is fed in accordance with a command from an operation panel(not shown). The document is delivered in accordance with a readtermination signal from the image reader 200. When a plurality ofdocuments are set, a control signal for these documents is consecutivelygenerated so as to efficiently feed, read and deliver the documents.

The image reader 200 is described. The document on a platen glass plate208 is illuminated by an exposure lamp 201. A light reflected from theilluminated document is guided by a group 202 of three mirrors to a lens203 so that the image is formed on a CCD sensor 204. The CCD sensor 204consists of three line sensors for red, green and blue arranged inparallel to each other, and the main scan direction is along the linesensors. The exposure lamp 201 and the first mirror scan the document atvelocity V in accordance with the magnification power in a directionindicated by an arrow by a scanner motor 209, thereby permittingscanning over the document on the platen glass plate 208. The second andthird mirrors are moved in the same direction at velocity V/2 with thescanning of the exposure lamp 201 and the first mirror. The position ofthe exposure lamp 201 is calculated and controlled in accordance with ascanner home sensor 210 and an amount of movement from home position(the number of steps of the motor). The light reflected from thedocument, which is incident on the CCD sensor 204, is converted to anelectric signal in the sensor. An image processing circuit 205 processesthe analog electric signal and converts it to digital image data. Then,the digital image is sent to the interface 207 and the image former 300.A white shading correction plate 209 is located at a different positionfrom the image read position of the platen glass plate 208. The shadingcorrection plate 209 is read in order to create correction data forshading correction prior to the read of image information on thedocument.

Next, the image former 300 is described. First, exposure and imaging isdescribed.

The image data transmitted from the image reader 200 or the interface207 is converted to print data of cyan (C), magenta (M), yellow (Y) andblack (K). The data is sent to controllers (not shown) of exposureheads. Each exposure head controller allows a laser to emit the light inaccordance with the electric signal of the transmitted image data. Theemitted light is scanned by a polygon mirror 301 in one dimension.Photoconductors in imaging units 302 c, 302 m, 302 y and 302 k areexposed to the light. Elements required for an electrophotographyprocess are arranged around the photoconductor in each imaging unit. Thephotoconductors for C, M, Y and K rotate clockwise, whereby theprocesses of image forming are consecutively performed. The imagingunits needed for these image forming processes are integrated for eachprocess, and they are detachable from a body. A latent image on thephotoconductor in each imaging unit is developed by each colordeveloping unit. A toner image on the photoconductor is transmitted topaper on a paper feeding belt 304 by a transfer chargers 303 c, 303 m,303 y, 303 k located opposite to the photoconductor in the paper feedingbelt 304.

Next, paper feeding and fixing is described. A sheet of paper, on whichthe image is to be transferred, is supplied to a transfer position andthe image is formed on the paper in the following sequence. Sheets ofpaper of various sizes are set in a group of paper cassettes 310 a, 310b and 310 c. The paper of a desired size is supplied to a feeding pathby a paper feeding roller 312 attached to each of the paper feedingcassettes 310 a, 310 b and 310 c. The paper supplied to the feeding pathis sent to the paper feeding belt 304 by a pair of feeding rollers 313.A reference mark on the paper feeding belt 304 is detected by a timingsensor 306 so that the paper is timely fed. Resist correcting sensors312 (three sensors along a main scan direction) are located at the mostdownstream position of the imaging units. When a resist pattern on thepaper feeding belt 304 is formed, these sensors detect the amounts ofhorizontal and vertical color shifts of C, M, Y and K images, and aprint image controller (PIC) corrects a drawing position and an imagedistortion, thereby preventing the color shift of the C, M, Y and Kimages on the paper. The toner image transferred on the paper is heated,melted and fixed on the paper by a fixing roller pair 307. Then, thepaper is discharged to a tray 311.

For a double-sided copy, the paper having the image fixed by the fixingroller pair 307 is reversed by a paper reversing unit 309 in order toform the image on the back side of the paper. The paper is guided by adouble side unit 308 and again fed therefrom. The paper feeding belt 304can be withdrawn from the C, M and Y imaging units in accordance with abehavior of a belt withdrawing roller 305. Thus, the paper feeding belt304 can be in no contact with the photoconductor. In forming amonochrome image, the C, M and Y imaging units can stop driving. It istherefore possible to reduce the wear of the photoconductor and theprocesses around the photoconductor.

FIG. 2 shows a structure of a laser optical system (LD head) includingthe polygon mirror 301 in top view. Four beams are reflected from thepolygon mirror 301. When the photoconductor for each color is exposed tothe light, C and M, from the laser, the colors to be painted on theupstream side are thus exposed and scanned in the direction opposite tothe direction in which the colors, Y and K, to be painted on thedownstream side are exposed and scanned. As described below, the printimage controller performs mirror image processing in the direction inwhich two colors on the upstream side are scanned, thereby solving thisproblem.

Next, a signal processing by the image reader 200 is described. FIGS. 3Aand 3B show a general block diagram of the image processor 205 in theimage reader 200. The light reflected from the document surface isformed by a reduction optical system onto an image on the CCD sensor204, and an analog signal is photoelectrically converted to image dataof color information of R, G and B. An A/D converter 401 converts theimage data of 400 dpi to 8-bit digital data (256 gradation levels) foreach color information of R, G and B by the use of the A/D converter inaccordance with a timing signal transmitted from a reference drive pulsegenerator 411.

In a shading corrector 402, the data obtained by reading the whiteshading correction plate 209 before reading the document have beenstored as reference data in an internal shading memory, independentlyfor each of R, G and B, in order to eliminate a variation in lightquantity in the main scan direction in which R, G and B data are scannedin the main scan direction. When the document is scanned, the data isconverted to a reciprocal thereof and then multiplied with the read dataof the document information, whereby the data is corrected on shading.

In an interline corrector 403, each color data is delayed line by lineby the use of an internal field memory in accordance with a scanvelocity (depending on the magnification power of subscan). Then, theread positions of the sensor chip for R, G and B are adjusted in thedirection in which each sensor chip for R, G and B is scanned.

Due to chroma aberration induced by an optical lens, a phase differenceamong R, G and B is larger at positions closer to the ends of a documentin the main scan direction. This influence causes an error in ACSjudgment and black character discrimination to be described below,besides the above-mentioned color shift. Therefore, a chroma aberrationcorrector 404 corrects the phase difference among R, G and B inaccordance with chroma information.

In a variable-magnification/displacement processor 405, two linememories for variable magnification are used for each of the R, G and Bdata so as to alternately input and output the data line by line, andthe write/read timing is independently controlled. Then, themagnification power and displacement in the main scan direction arechanged. That is, the reduction is accomplished by thinning the datawhen the data is written to the memory, while the enlargement isaccomplished by increasing a number of the data when the data is readfrom the memory. Under this control, the interpolation is performed inaccordance with the magnification power. For the reduction, theinterpolation is performed before the data is written to the memory. Forthe enlargement, the interpolation is performed after the data is readfrom the memory. This prevents defects and distortion of the image.Besides the enlargement and the reduction, a combination of the controlby this block and the scan control realizes centering of the image,image repeating, consecutive enlargement, reduction in a binding margin,or the like.

In a histogram generator 412 and an automatic color selector (ACS) 413,value data of the document is generated from the R, G and B dataobtained by prescan, and the histogram of the value data is created in amemory (histogram memory), before the document is copied. Meanwhile, itis judged in accordance with chroma data whether or not each dot is acolor dot, and the number of color dots in each mesh of 512 dots in thedocument is counted in the memory (ACS memory). Automatic control ofcopy background level (automatic exposure processing) and automaticcolor selection (ACS) for judging whether a color copy operation or amonochrome copy operation is selected are performed in accordance withthis result.

A line buffer 414 has memories which can store one line of the R, G andB data that are read by the image reader 200. The line buffer 414monitors the image data for image analysis for automatic sensitivitycorrection and automatic clamp of the CCD sensor for the A/D converter401.

In a bill recognizer 415, R, G and B data areas are selected at any timeto prevent a normal copy when a security such as a bill (paper money) isput on the platen glass plate 208. It is judged by pattern matchingwhether or not the document is a bill. As soon as the document is judgedas a bill, a central processing unit for controlling the read operationby the image reader 200 and the image processor 205 outputs a blackpaint signal (−PNT=“L”) to the print image controller. (The “−” before areference sign means a signal of negative logic.) The print imagecontroller switches K data to black paint so as to inhibit a normalcopy.

An HVC converter 422 temporarily converts the R, G and B data receivedfrom a data selector 421 to value (V data) and color difference signals(Cr and Cb data) by a 3*3 matrix operation.

Then, an automatic exposure (AE) processor 423 corrects the V data inaccordance with the background level control value and corrects the Crand Cb data in accordance with chroma level and hue level which are setby the operation panel. Then, an inverse HVC converter 424 reconvertsthe V, Cr and Cb data to the R, G and B data by an 3*3 inverse matrixoperation.

In a color corrector, a logarithm corrector 431 converts the R, G and Bdata to gradation level data (DR, DG and DB data). Then, a blackextractor 432 detects a minimum color level of the DR, DG and DB data asunder color component. At the same time, a black extractor 432 detectsgradation level difference between maximum and minimum colors of the R,G and B data as document chroma data.

The DR, DG and DB data are subjected to a 3*6 nonlinear matrix operationin a masking operation processor 433. The DR, DG and DB data areconverted to the color data (C, M, Y and K data) which match colortoners of the printer.

An under color removal/black paint processor (UCR/BP processor) 434calculates UCR/BP coefficients of the under color component (Min(R, G,B)) in accordance with the document chroma data. The UCR/BP processor434 determines the amounts of UCR and BP by multiplication. The UCR/BPprocessor 434 determines a finite difference in the amount of undercolor removal (UCR) among the C, M and Y data from the C, M and Y datawhich are subjected to the masking operation. The UCR/BP processor 434calculates the C, M and Y data and then calculates the BP amount equalto K data. A monochrome data generator 435 creates a value componentfrom the R, G and B data, corrects the data with logarithm operation andoutputs the data as black data (DV data). Finally, a color data selector436 selects the image for color copy, i.e., the C, M, Y and K data orthe image for monochrome copy, i.e., the DV data (C, M and Y are white).

A region discriminator 441 detects the difference (Max(R, G, B)−Min(R,G, B)) between the minimum color (Min(R, G, B)) and the maximum colorfrom the R, G and B data which are inputted to the region discriminator441 through a data selector 442. Then, a black character, a colorcharacter, a dot, and the like are discriminated. The regiondiscriminator 441 corrects a character edge for the black characterdiscrimination and transmits the data, as well as the result of thediscrimination, to a character edge reproducer 451. At the same time, itgenerates and transmits an attribute signal for switching a method ofreproducing the gradation level to the print image controller and to aprint head controller.

The character edge reproducer 451 performs the correction (edgeemphasis, smoothing and character edge removal) suitable for eachdiscriminated region from the C, M, Y and K data received from the colorcorrector in accordance with the result of the region discrimination.Finally, a sharpness/gamma/color balance adjuster 452 corrects the imageof the C, M, Y and K data in accordance with sharpness level, colorbalance level and gamma level specified with the operation panel. Theadjuster 452 transmits a gradation reproduction attribute signal,−LIMOS, to a print image control interface 453. The adjuster 452 sendsthe C, M, Y and K data through a data selector 461 to an image interface462.

The image interface 462 allows the image to be inputted and outputtedto/from an external apparatus. The image interface 462 is operated sothat it can simultaneously receive and send the R, G and B data and itcan sequentially receive and send the C, M, Y and K data. The externalapparatus can use the color copying machine for the scanner function andfor the printer function.

The system explained above is a copying machine for simultaneouslyoutputting four colors per scan. FIGS. 4A and 4B show relationshipbetween the system structure and the print image controller block. Asshown in FIGS. 4A and 4B, the C, M, Y and K data from the image reader200 are simultaneously transmitted to the print image controller in onescan operation. Accordingly, the processing in the print imagecontroller is basically parallel operation for the C, M and Y data. Thesystem needs to transmit the image composed of C, M, Y and K tonercomponents on the paper fed onto the paper feeding belt 304 without thecolor shift.

However, as shown schematically in FIGS. 5A-5F, the color shifts happendue to various factors. Because the photoconductors are substantiallyequally spaced above the paper feeding belt 304, the timings fordeveloping the C, M, Y and K toners are shifted by the times dependingon distances between the photoconductors. The timings for C, M, Y and Kare therefore controlled by using subscan delay modules so that they aredelayed by the amounts depending on the distance between thephotoconductors in the subscan direction. However, as shown in FIG. 5A,the colors are deviated when for example C is shifted and drawn in thesubscan direction. Moreover, the latent images are formed on thephotoconductors with laser scan by using one polygon mirror and fourbeams. Thus, a raster scan direction of the first two colors (C and M)is opposite to that of the latter two colors (Y and K) and, as a result,the colors are shifted due to this mirror image relationship (FIG. 5F).In addition, for example, the print start positions are displaced in themain scan direction due to the laser scan of the colors (FIG. 5E), themagnification power in the main scan direction is distorted (FIG. 5D), abow distortion takes place in the subscan direction (FIG. 5C), a skewdistortion takes place due to the difference in parallelism between thearrangement of the photoconductors and the laser scan (FIG. 5B), andthese factors also cause the color shift. These phenomena are correctedby correcting the position and the images for the C, M, Y and K data, inorder to prevent the color shift.

It is the print image controller shown in FIG. 6 that performs thesecorrections. First, the C, M, Y and K image data transmitted from theimage processor 205 are inputted to a gradation reproducer 500. Here,the 8-bit gradation level of the C, M, Y and K data is converted to3-bit pseudo 256-gradation-level data by a multi-level error diffusionof character separation type in accordance with the −LIMOS signal(gradation reproduction attribute signal). Then, a drawing positioncontroller 510 corrects the position, at which the K component is drawnby the K developing unit located at the most downstream position, on thebasis of the paper, in order to correct the shift of the developingtiming depending on the distance between the photoconductors (see FIG.1). The drawing position controller 510 corrects the positions of theother color components for the subscan with respect to the K component.Next, in order to correct the difference in the laser scan direction andthe shift of the main scan start position, the C and M images aresubjected to main scan mirror image processing. As in the case of thesubscan, the position of the K component is corrected on the basis ofthe paper, while the positions of the other color components arecorrected with respect to the K component. A frame memory 520 needs topreviously store the image on the front side at the time of adouble-sided copy. For this reason, the frame memory 520 has a memoryunit whose capacity is equivalent to one surface of A3-size paper, forstoring the data from the gradation reproducer 500.

Test pattern data of C, M, Y and K for resist detection, which aregenerated by a test data generator 530, are simultaneously transferredonto the paper feeding belt 304. The amounts of color shift of the C, Mand Y components from the K component are detected by a resist detectingsensor 312 which is located in a still more downstream site than the Kdeveloping unit located at the most downstream position. An imagedistortion corrector 540 corrects the main scan magnificationdistortion, the subscan bow distortion and the skew distortion of the C,M and Y components by means of the interpolation by a gradation leveldistribution in accordance with the result of the color shift detection,as diagrammatically shown in FIG. 7. The K image data alone from thedrawing position controller 510 is decoded to the 256-gradation-leveldata. The K image data or black paint data is selected in accordancewith the result of the above-described bill recognition. The C, M, Y andK data, which are finally corrected, are shifted to the image positionbased on the paper by the print imaging controller/print head controllerinterface. The C, M, Y and K data are transmitted to the print headcontroller, and then the image is reproduced. A reference positionsignal generator 550 generates various reference position signals inaccordance with various input signals.

FIG. 8 shows the print head controller, wherein a gradation leveldistortion due to gamma-characteristic in the electrophotography processis corrected by a gamma-correction table 320. Then, the gradation levelis converted by a D/A converter 321 to an analog level. The mostsignificant bit of each color corresponds to −LIMOS signal (thegradation reproduction attribute signal) transmitted from the imagereader. Light modulation method is switched by a switch 326 inaccordance with the gradation reproduction attribute signal. When themost significant bit is “L” (=character edge), an LD drive signal isgenerated by pulse width modulation (PWM) in which a pulse having acycle of one dot is compared to a reference signal 322 for one dot by acomparator 323. When the most significant bit is “H” (=continuousgradation level), the LD drive signal is generated by pulse widthmodulation (PWM) in which a pulse having a cycle of two dots is comparedto a reference signal 324 for two dots by a comparator 325. By driving asemiconductor laser as explained above, the image is exposed to thelight on the photoconductors so as to express the gradation level. Forthe 2-dot pulse width modulation, a screen angle of 45° is set so thatthe granular characteristic of the image may be improved. The characteredge is reproduced by the 1-dot pulse width modulation which givespriority to resolution and has no defects in characters. For the otherregions, the gradation level reproduction is automatically performed.This gradation level reproduction is resistant to an image noiseproduced by the 2-dot pulse width modulation and the 45-degree screenangle modulation, thus forming the smooth image having excellentgranular characteristic.

As is described in detail later, optimum image correction is performedby the character edge reproducer 451 in accordance with the regiondiscrimination result obtained by the image reader 200. The gradationreproducer in the print image controller switches the processing to themulti-level error diffusion or simple quantization depending on thejudgment on whether or not the region is the character edge inaccordance with the gradation level attribute signal. Finally, the printhead controller automatically switches the light modulation on thephotoconductor so as to improve the quality of the image.

FIGS. 9A to 9C show the color corrector. FIG. 9C shows the signal to besupplied to the portion shown in FIG. 9B. The color correctionrepresents generally image correction performed in the LOG corrector431, the black quantity extractor 432, the masking operation processor433, the black print/under color removal processor 434 and the colordata selector 436. In a negative/positive reversing unit 601 in thecolor corrector, the R, G and B input data, R, G and B₈₇₋₈₀, arecontrolled in accordance with a status of a negative/positive reversalarea signal −NPAREA which a central processing unit sets through a colorcorrection controller, and the output data, R, G and B₉₇₋₉₀, arecontrolled in the following manner, whereby the negative/positivereversal is performed.−NPAREA=“L”→R, G, B ₉₇₋₉₀=255−R, G, B ₈₇₋₈₀,and−NPAREA=“H”→R, G, B ₉₇₋₉₀ =R, G, B ₈₇₋₈₀.

Next, because the R, G and B data are the signals to be changed linearlywith respect to reflectance of a document, the R, G and B data areinputted to an R, G, B-LOG_TABLEs 602 and transformed to gradation leveldata DR, DG and DB₇₋₀ which respond linearly to the change in thegradation level. The transformation is expressed as the followingequation:D ₇₋₀={−log(Wh*(A ₇₋₀/256))−D _(min)}*256/(D _(max) −D _(min)),where D_(max) denotes a maximum gradation level range, D_(min) denotes aminimum gradation level range and Wh denotes the reference reflectanceof the document in the shading corrector 402.

In order to generate a gradation level signal V₉₇₋₉₀ for the monochromereproduction, a value generator 603 calculates the gradation levelsignal V₉₇₋₉₀ from the R, G and B data in the accordance with thefollowing equation:V ₉₇₋₉₀ =Ra*R ₉₇₋₉₀ +Ga*G ₉₇₋₉₀ +Ba*B ₉₇₋₉₀,where Ra, Ga and Ba denote parameters of R/G/B mixture ratio to be setby a color correction controller 604. Ra, Ga and Ba are generally set toRa=0.3, Ga=0.6 and Ba=0.1, thereby providing the value data close to thedistribution of spectral luminous efficiency.

V₉₇₋₉₀ is inputted to a V-LOG_TABLE 605 and transformed to gradationlevel data DV₇₋₀ for the purpose of logarithm correction, as in the caseof the R, G and B data. DV₇₋₀ is multiplied by data MC, MM, MY and MK₇₋₀on ratios of C, M, Y and K for determining color separation data C, M, Yand K₁₇₋₁₀ for the monochrome reproduction. For example, red monochromereproduction can be accomplished by setting MC₇₋₀=MK₇₋₀=0 andMM₇₋₀=MY₇₋₀=128.

On the other hand, the difference between the maximum and minimum colorsof the R, G and B data is calculated by circuits 606 to 608. Thedifference is inputted as chroma data W₉₇₋₉₀ to UCR/BP_TABLEs 609 and610. These tables are used to control the amount of under color removaland the amount of black paint for the UCR/BP processing in accordancewith the status of W₉₇₋₉₀. Here, the under color removal is operated.That is, the minimum color (Min(DR, DG, DB)) is detected from the DR, DGand DB data with logarithm correction by a minimum value circuit 611.The detected data is defined as a basic black paint quantity. A certainratio (BP quantity) is treated as the K data. Black toner is added tothe printer by multipliers 612 and 613 (black paint), and a certainratio of the basic black quantity (UCR quantity) is subtracted from theC, M and Y data which have been subjected to the masking operation fromDR, DG and DB. The outputs of the UCR/BP_TABLEs 609 and 610 controlthese ratios and are defined by a function of W₉₇₋₉₀. If the output ofthe UCR_TABLE is α(W), the output of the BP_TABLE is β(W) and the K datadifference SB₇₋₀ from the color correction controller is k, thefollowing equations hold.UCR ₇₋₀(UCR quantity)=MIN(DR, DG, DB)*α(W)/256,andBP ₇₋₀(BP quantity)=MIN(DR, DG, DB)*β(W)/256−.kThe difference is determined by a subtractor 614. When the read R, G andB data are achromatic color (white and black), i.e., when the chromasignal W₉₇₋₉₀ is low, the printer had better reproduce the color by theuse of the K toners alone, because less toner adheres to the paper andthus the print image looks more black and sharp. In such a case, it istherefore desirable that the value α(W)/β(W) is increased whereby theUCR/BP quantities are increased. On the contrary, in the case ofchromatic color, the large values α(W) and β(W) yield dull colorreproduction. In other words, the high chroma signal W₉₇₋₉₀ would reducethe value α(W)/β(W). As described above, α(W) and β(W) are used for theoptimum control in the UCR_TABLE 609 and the BP_TABLE 610, respectively,in accordance with the chroma signal W₉₇₋₉₀.

At the same time when the basic black quantity MIN(DR, DG, DB) iscalculated, the DR, DG and DB maximum color data MAX(DR, DG, DB) is alsocalculated by a maximum value circuit 615. This data MAX₇₋₀ istransmitted to the following character edge reproducer. The data MAX₇₋₀is used as black character correction data which replaces the Kcomponent as the BP quantity in the black character discriminationregion. The DR, DG and DB₇₋₀ subjected to logarithm correction aretransmitted to the masking operation processor 433 in order that theyapproximate to the spectral distribution of the color toner of theprinter and thus improve color reproducibility.

In the masking operation processor 433, a nonlinear masking, which usesthe nonlinear terms of DR, DG and DB, i.e., DR*DG, DG*DB and DB*DR, isperformed by using masking coefficients inputted from the colorcorrection controller 604 in order to improve approximate accuracy. TheDR, DG and DB data are transformed into the C, M and Y data.$\begin{matrix}{\begin{pmatrix}C \\M \\Y\end{pmatrix} = {\begin{pmatrix}a_{11} & a_{12} & a_{13} & a_{14} & a_{15} & a_{16} \\a_{21} & a_{22} & a_{23} & a_{24} & a_{25} & a_{26} \\a_{31} & a_{32} & a_{33} & a_{34} & a_{35} & a_{36}\end{pmatrix}*\begin{pmatrix}{DR} \\{DG} \\{DB} \\{{DR}*{{DG}/256}} \\{{DG}*{{DB}/256}} \\{{DB}*{{DR}/256}}\end{pmatrix}}} & (1)\end{matrix}$The masking coefficients are experimentally determined in such a mannerthat a test color print formed by the image former 300 is read by theimage reader 200 and the data difference between the image former 300and the image reader 200 is minimum.

In the UCR processor 434, the UCR quantity is then subtracted from theC, M and Y data subjected to the masking operation. The resultant datais then outputted together with BP quantity=K data as color separationdata C, M, Y and K₂₇₋₂₀ for the color copy. Thereafter, in the case of amonochrome image area (−MCAREA=“L”), the color data selector 436replaces the C, M, Y and K data by color separation data C, M, Y andK₁₇₋₁₀ for the monochrome reproduction. In the case of an image erasearea (−CLRAREA “L”), the color data selector 436 replaces the C, M, Yand K data by “0”. When the input of the sequential C, M, Y and K datatransmitted from the image interface 462 is selected (−IFSEL1=“L”), theC, M, Y and K data are replaced by IFD₇₋₀. Then, color separation dataC, M, Y and K₃₇₋₃₀ are transmitted together with the black charactercorrection data MAX₇₋₀ to the character edge reproducer 451.

FIGS. 10A and 10B show the region discriminator 441. The R, G and Bdata, R, G and B₈₇₋₈₀, transmitted from the inverse HVC converter 424are subjected to the region discrimination such as the black characterdiscrimination, the color character discrimination, the dotdiscrimination and the switching of the gradation level reproduction.

First, extraction of the value and chroma components required for theregion discrimination is described. A minimum color Min₇₋₀ of input R, Gand B signals is used as the value component needed to detect thecharacter edge and to detect isolated dots during the dotdiscrimination. The minimum value Min₇₋₀ is obtained by a minimum valuecircuit 701. The minimum color Min₇₋₀ is used because discrimination isnot influenced by the colors in the document. For example, if thedocument has the black color character, the R, G and B signals havesubstantially the same low gradation level. However, if the document hasthe red color character, the R signal is bright and the G and B signalsare dark. Accordingly, when the minimum value of the R, G and B signalsis used, both the red and black characters depend only on a characterdensity and respond to substantially the same value level. Thus, thecharacter and the dot can be judged in accordance with the colors onvarious documents. A difference W₈₇₋₈₀ (i.e., Max(R, G, B)−Min(R, G, B))between the maximum and minimum colors of the R, G and B data obtainedby a maximum value circuit 702 is determined by a subtractor 703. Thedocument chroma (the judgment on whether or not the image is black)needed for the black character discrimination is judged based on thedifference W₈₇₋₈₀.

Next, the black character discrimination in the region discriminator 441is described. The contents of the black character discrimination aregenerally classified into the character judgment (edge judgment), theprevention of the erroneous black judgment and erroneous black characterdiscrimination, and the generation of a black edge correction signal.

First, the judgement of character (edge) is described. A value componentMin₈₇₋₈₀ is expanded in a two-dimensional matrix of 5 dots*5 lines bythe use of line memories 704 to 707. The value components of five linesare inputted to a primary differential filter 708, a secondarydifferential filter 709, a character/background boundary discriminator710, an isolated dot detecting filter 711, and a black edge correctingfilter 712.

In the primary differential filter 708, as shown in FIG. 11, the skew ofthe pixels adjacent to an objective pixel (central pixel) is detected inthe main scan and subscan directions independently of each other. Thevalue is calculated by adding absolute values thereof and is outputtedas a primary differential edge quantity FL₁₇₋₁₀. In the secondarydifferential filter 709, as shown in FIG. 12, a secondary differentialvalue of the objective pixel is determined independently of one anotherin the vertical, horizontal and diagonal directions. The maximum valueof the absolute values is outputted as a secondary differential edgequantity FL₂₇₋₂₀. Comparators 713 and 714 compare the edge quantitieswith edge reference data EDGREF₁₇₋₁₀ and EDGREF₂₇₋₂₀. If either edgequantity is larger than a reference level, −FLON=“1” is outputted as thecharacter edge through a buffer circuit 715.

In the character/background boundary discrimiator 710, as shown in FIG.13, an integral value (average value) of the secondary differentialfilters in four directions is determined. If the code is “L”, the edgeis discriminated as the character-side edge (hereinafter referred to asan inside edge). If the code is “H”, the edge is discriminated as thebackground-side edge (hereinafter referred to as an outside edge). Then,−FLAREA is outputted. The signal −FLAREA is delayed by a predeterminedline. The signal is outputted to the character edge reproducer as acharacter/background boundary discrimination signal −INEDGE.

Two differential filters are used for the character edge detection dueto the following reason. As shown in FIG. 14, the primary differentialfilter 708 is used to detect the gradation level difference between thepixels adjacent to the objective pixel. The filter 708 judges a regionnear the boundary between the line and the background as the edge. Thesecondary differential filter 709 is used to detect the sum of theobjective pixel and the gradation level differences between the pixelsadjacent to the objective pixel. The filter 709 detects the reversedcode in regions adjacent to the region near the boundary between theline and the background. In the case of a thin line, the filter 709detects the line itself as the edge. Accordingly, the combination of twotypes of filters enables the character edge to be judged continuouslyfrom a thin line to a thick line. Furthermore, a boundary can be judgedin accordance with a change in the code near a boundary between the edgeof secondary differential and the background.

The signal −FLON, which has once detected an edge, is transmitted to aclosing processor 716. In the closing, the result of −FLON=“L” (thecharacter-side edge) is first thickened by the use of the matrix of 3dots*3 lines. If the 3*3 matrix includes even one dot of −FLON=“L”, thisthickening forces the result of −FLON of the central dot to be replacedby “L”. In short, the result of −FLON=“L” is thickened by one dot for aprevious line and for a following line. In three examples shown in theuppermost portion in FIG. 15, there are shown one dot, two dots andthree dots of −FLON=“H” (the background-side edge) in the main scandirection. In these examples, the region of −FLON=“L” is increased byone dot with the thickening. Then, the result of the edge judgment afterthe thickening is again expanded in the matrix of 3 dots*3 lines,whereby the result is conversely thinned. This thinning forces theresult to be replaced by “H”, if the 3*3 matrix includes even one dot(−FLON=“H”) that is not an edge in the character side. In short,contrary to the thickening, the result of −FLON=“L” is thinned by onedot in the previous and following lines. The signal can be thusconverted to the character edge (−FLON=“L”), only when the dot is judgedas the dot (−FLON=“H”) that is not the character-side edge within thedistance of two dots or less in the main scan and subscan directions, asshown in FIG. 15.

One of the results is selected by a selector 717 in accordance with thecharacter/background boundary discrimination signal (−FLAREA), where theresults include a result after the character edge closing (delay by twolines on the matrix expansion) and a result obtained by simply delayingthe result before the character edge closing by two lines. That is, theclosed character edge judgment result is used as the final characteredge signal in the inside edge (−FLAREA “L”), while the non-closedcharacter judgment result is used as the final character edge signal inthe outside edge (−FLAREA=“H”). After a predetermined line delay, a −EDGsignal is transmitted to the character edge reproducer 451.

This processing is performed for the following purpose. Edge isemphasized on a pixel judged as the character edge by the character edgereproducer 451 as described below. At this time, the edge emphasisallows a character to be bordered, thereby enhancing the contrast.However, when a character has a thickness of about 5 to 7 dots, at thecenter of the character, there are pixels of one or two dots that arenot subjected to edge emphasis. This gives an impression that thereproduced character is hollow (see FIG. 16). The judgment is thereforecorrected. That is, only an inside edge is judged as the character edge,so that edge emphasis is performed on the pixels at the character edgesurrounded by two or less dots in the main scan and subscan directions.Thereby, the above-mentioned phenomenon is eliminated, and thus thereproducibility of characters is improved. However, if this correctionis provided to an outside edge, the image in the region between thelines such as a line pair is vanishes.

Next, the black judgment in the region discriminator 441 is described.Whether or not the image is black is determined, by comparing thedifference W₈₇₋₈₀ between the maximum and minimum colors of the R, G andB data with chroma reference data WREF₇₋₀ which the minimum color dataMIN₈₇₋₈₀ is converted to by a chroma reference table 720. As shown inFIG. 17, the chroma reference table 720 has a low reference at the lowand high value levels and has zero above a certain value level. In thecomparison of the reference data to W₈₇₋₈₀, the level at which the imageis judged as black can be varied depending on the value level. Thus, theimage is not judged as black at all above the certain value levelbecause this level represents the background. Thereby, the table 720deals with the increase in the chroma difference due to a little phasedifference among the R, G and B data in a character edge (atintermediate gradation levels), while it judges the value levelcollectively. Although this table is set by a central processing unit,its contents can be varied in accordance with a background leveladjustment value obtained by the foregoing automatic exposureprocessing. The comparison between WREF₇₋₀ and W₈₇₋₈₀ is performed by acomparator 721. If the image is black (WREF₇₋₀>W₈₇₋₈₀), −BK=“L” isoutputted. After the four-line delay for the delay quantity of the edgedetection, if the result of the character edge judgment after theclosing is “L”, the character edge is once judged as a black characteredge, or −BKEG=“L”.

Next, the prevention of erroneous black character judgment in the regiondiscriminator 441 is described. When the black character discriminationalone is performed, a character edge having low chroma (green, royalpurple, etc.) may be misjudged. Therefore, when color dots of uniformdensity are detected, if the central pixel in the area having a largenumber of color dots of uniform density is judged as a black character,the result is canceled. First, W₈₇₋₈₀ is compared by a comparator 722with chroma reference data WREF₁₇₋₁₀ set by a central processing unit.If the chroma is high (W₈₇₋₈₀>WREF₁₇₋₁₀), WH is set to “L”. A comparator723 compares MIN₈₇₋₈₀ to value reference data VREF₁₇₋₁₀ set by thecentral processing unit. If the value is low (MIN₈₇₋₈₀<VREF₁₇₋₁₀), −VLis set to “L”. If a pixel has −VL=“L” and WH=“L” and is judged as anon-edge (−FLON=“H”) by the edge detection, it is judged as a colorsolid pixel, −CAN=“L”.

The data are expanded in a matrix of 9 dots*9 lines by the circuit 714,and the number of dots of −CAN=“L” in the matrix is determined. Thevalue CANCNT₇₋₀ is compared, by a comparator 725, to a count referencevalue CNTREF₁₇₋₁₀ set by the central processing unit. If the number ofcolor dots of uniform density is more than the count reference value(CANCNT₇₋₀>CNTREF₁₇₋₁₀), -BKEGON is set to “H”. The result (−BKEGON=“L”)of the black character discrimination, which has been once judged, iscanceled. On the other hand, if the number of color dots of uniformdensity is less than the count reference value, the result is allowedand transmitted to the character edge reproducer as the final result(−PAPA=“L”) of the black character discrimination.

Next, the generation of the black edge correction signal in the regiondiscriminator 441 is described. As shown in FIG. 18, the black edgecorrecting filter 712 inputs the minimum color MIN₈₇₋₈₀ of the R, G andB data to the secondary differential filters in four directions. Theresults of the filters obtained independently are clipped to “0” (anegative value is clipped to “0”). The maximum value of each directionalresult is inputted as black edge correction data FL₃₇₋₃₀ to a black edgecorrection table 731 by a maximum value circuit 730. The result of theconversion by the table is taken as VEDG₇₋₀. After a predetermined linedelay, the result is outputted as the edge correction data for the blackcharacter to the character edge reproducer. The data is used as anamount of the edge emphasis of the black character edge. The maximumvalue of the four secondary differential filters is used as the edgecorrection data in order to improve a generation characteristic of theblack character copy. As exemplified in FIG. 19, when an edge isenhanced by the secondary differential filter of 45°, the lines arethinned at an intersection of the lines crossing at 90°. If the copiesare repeatedly reproduced through generations (i.e., a master copy (afirst generation) is copied to obtain a new copy (a second generation),the obtained copy (the second generation) is then copied to obtainanother new copy (a third generation), and the third generation and thefollowings are repeatedly reproduced in the same manner), the lines arenoticeably thinned at crossing points and thus the character isdifficult to be read. When the edge is enhanced by the secondarydifferential filter of 0°/90°, the intersection of the lines crossing at90° is lost. This is also not preferable from the viewpoint of thegeneration characteristic. The maximum value of the result of the fourfilters is used as the edge correction quantity, in order to prevent theimage deterioration caused due to this phenomenon.

The edge correction signal for the black character is determined fromthe minimum color of the R, G and B data due to the following reason.Since R, G and B are value information, a edge change quantity of the R,G and B data in a filter is more sensitive to the background level(white background) on which the more intense character edge emphasis isneeded, while it is less sensitive to a high gradation level on whichthe less intense enhancement is needed than that of the C, M, Y and Kdata subjected to logarithm correction.

The black edge correction table 731 is provided in order to convert thefilter data FL₃₇₋₃₀ so that the amount of edge emphasis may be a propervalue in the black character edge emphasis. The contents of the table731 are set by the central processing unit.

A Laplacian filter used generally in the edge emphasis is an inversefilter of the secondary differential filter. The image data subjected toedge emphasis by the character edge reproducer is the gradation leveldata of C, M, Y and K. This data is the inverse of the value dataMIN₈₇₋₈₀ (having the opposite white/black gradation levels). Thus, thesecondary differential filter can be used.

Next, the dot discrimination in the region discriminator 441 isdescribed. The minimum color MIN₈₇₋₈₀ of the R, G and B data is inputtedto the isolated dot detecting filter 711 in the same manner as the edgedetection. As shown in FIG. 20, MIN₈₇₋₈₀ is expanded in a matrix 741 of5 dots*5 lines. Then, an isolated dot condition decision section 742judges whether or not each pixel is an isolated dot having the sameimage distribution as that of the central pixel of the dots in a dotprint.

The isolated dot detecting filter 711 judges whether or not the pixelsatisfies two types of isolated dot conditions, in order to judgewhether the pixel is a valley (white isolated dot) or a peak (blackisolated dot) in the dot print.

First Condition: The gradation level of a central pixel X₃₃ is higher(white isolated dot condition) or lower (black isolated dot condition)than the gradation levels of eight peripheral pixels around the centralpixel X₃₃:X ₃₃≧MAX(X ₁₂ , X ₂₃ , X ₂₄ , X ₃₂ , X ₃₄ , X ₄₂ , X ₄₃ , X ₄₄),andX ₃₃≦MIN(X ₁₂ , X ₂₃ , X ₂₄ , X ₃₂ , X ₃₄ , X ₄₂ , X ₄₃ , X ₄₄).Second Condition: The gradation level of a central pixel X is higher(white isolated dot condition) or lower (black isolated dot condition)than an average level of the gradation level distributions in eightperipheral directions:X ₃₃>MAX(X ₁₁ +X ₁₂ , X ₁₃ +X ₂₃ , X ₁₅ +X ₂₄ , X ₃₁ +X ₃₂ , X ₃₄ +X ₃₅, X ₅₁ +X ₄₂ , X ₅₃ +X ₄₃ , X ₅₅ +X ₄₄)/2+AMIREF ₇₋₀;andX ₃₃>MIN(X ₁₁ +X ₁₂ , X ₁₃ +X ₂₃ , X ₁₅ +X ₂₄ , X ₃₁ +X ₃₂ , X ₃₄ +X ₃₅, X ₅₁ +X ₄₂ , X ₅₃ +X ₄₃ , X ₅₅ +X ₄₄)/2−AMIREF ₇₋₀,where isolated dot reference data AMIREF₇₋₀ for determining the isolateddot conditions is an image parameter set by the central processing unit.The pixel, which satisfies two types of conditions described above, istransmitted to the following step as the white isolated dot (−WAMI=“L”)or the black isolated dot (−KAMI=“L”).

Two types of isolated dot information is then expanded into the matrixof 41 dots*9 lines. As in the case where the number of “L” dots of a−CAN signal is counted for the prevention of the black charactermisjudgment, counters 743 and 744 count the number of “L” dots of −WAMIand −KAMI. The count values are WCNT₇₋₀ and KCNT₇₋₀ (if the count valuesare 255 or more, they are clipped to 255). The data WCNT₇₋₀ and KCNT₇₋₀as to the number of white and black isolated dots are compared inparallel, by comparators 745 and 746, to reference data CNTREF₂₇₋₂₀ asto the number of isolated dots. If WCNT₇₋₀ or KCNT₇₋₀ is larger thanCNTREF₂₇₋₂₀, the image is judged as the dot print image and AMI1=“L” isoutputted. That is, a condition of the dot image discrimination is thatthe number of the pixels (−WAMI=“L” or −KAMI=“L”), which have the sameimage distribution as that of the dots in the dot image, is equal to orlarger than a fixed value CNTREF₂₇₋₂₀ in a certain unit area (41 dots*9lines).

A rough value of the reference CNTREF₂₇₋₂₀ as to the number of isolateddots is described. The image read condition of the system is 400 dpi.Assuming that the dot print conditions are that the screen angle is 45°and that the number of screen lines is W, at least 2*(W/SQRT(2))² dotsare thus present in an area of one inch square (400*400 dots).Therefore, CNTREF₂₇₋₂₀ is expressed as the following equation.CNTREF ₂₇₋₂₀=(369/160,000)*W ².If W=100, the reference value is 23. This value is obtained when theisolated dot detecting filter 711 ca detect the dot pixels with accuracyof 100%. Thus, the value which is a little lower than the calculatedvalue is, in fact, a proper value. It is necessary to change thereference value, depending on the magnification power or the like. Forthe enlargement, the value CNTREF₂₇₋₂₀ is smaller than CNTREF₂₇₋₂₀ for alife size reproduction because of a small number of isolated dots perunit area. On the contrary, CNTREF₂₇₋₂₀ for the reduction is larger thanCNTREF₂₇₋₂₀.

It is difficult for the isolated dot detecting filter 711 to judge a dotpixel as an isolated dot, in the case of a print image having a smallnumber of screen lines of the dots (a great distance between the dots)and a dot-area ratio of about 50%. A particular case is theintermediate-gradation-level dot print having about 65 to 85 screenlines. When the document to be printed has the intermediate gradationlevel, the isolated dot detecting filter 711 judges that the white andblack isolated dots are substantially equally mixed because the dot-arearatio is about 50%. Consequently, the number of white isolated dots issubstantially equal to that of black isolated dots. Thus, the value doesnot reach to the above-mentioned value CNTREF₂₇₋₂₀. Thus, previously,the pixel has or has not been judged as a dot at about the intermediategradation levels in the dot print. This may produce image noises. Inorder to solve this problem, the following processing is additionallyperformed. First, a sum of the number of white isolated dots WCNT₇₋₀ andthat of black isolated dots KCNT₇₋₀ is determined. Then, the sum iscompared to another reference data CNTREF₃₇₋₃₀ as to the number ofisolated dots by a comparator 747, thereby judging whether or not thedocument is a dot print image.

After it is once judged whether a pixel is a dot (−AMI=“L”) or not(−AMI=“H”), a −AMI1 signal is inputted to eight types of delay blocks.Under delay control by predetermined lines and dots, if any one of thedot results −AMI₁₋₉ is “L”, i.e., the dot, the document is judged as adot print, and −AMIOUT=“L” is transferred to the subsequent characteredge reproducer. This means, as shown in FIG. 21, that whether or notone of the numbers of isolated dots exceeds a certain level is judged ina region shifted according to the central pixel to be judged by acertain number. Therefore, even if a dot print portion is included in adocument, the accuracy of the dot discrimination is not decreased near aboundary of the dot print portion.

The processing in the region discriminator 441 has been described above.In the block diagrams shown in FIGS. 10A and 10B, signals needed for thediscriminations are synchronized to one another. Thus, the delay controlby predetermined numbers of lines or dots is performed. For example, forthe dot discrimination, the discrimination result −AMIOUT is delayedwith respect to the input R, G and B data by ten lines in total, twolines by the line memory, four lines by the count of the isolated dots,and four lines for shifting the discrimination region from the centralpixel. For the black character discrimination, the discrimination result−PAPA is delayed with respect to the input R, G and B data by ten linesin total: two lines by the line memory, two lines by the closing, twolines for synchronizing the count result of the −CAN signal forpreventing the misjudgment, and four lines for synchronizing the dotresult.

In this manner, the discrimination results of character edgediscrimination signal −EDG, black character discrimination signal −PAPA,character/background boundary discrimination signal −INEDG, dotdiscrimination signal −AMIOUT and black edge correction signal VEDG₇₋₀,are delayed so as to prevent phase shift at the output positions. Theresults are transmitted to the subsequent character edge reproducer 451.

FIGS. 22A and 22B show the character edge reproducer 451. The characteredge reproducer 451 performs appropriate image correction on the C, M, Yand K data after color correction in accordance with the result of thediscrimination in the region discriminator 441. Although C, M, Y and Kare processed in parallel, the C, M and Y signals are processed in thesame manner, while the K signal is differently processed. The regiondiscrimination result is inputted to a character edge reproductioncontroller 801. The result is converted to a select signal for switchingthe corrections in the character edge reproducer 451. The contents ofthis conversion are changed in accordance with the status of documentmode signal MODE₃₋₀ and monochrome image area signal −MCAREA which areinputted together with the region discrimination result. The documentmode signal is used in order that a user specifies a document on theplaten glass plate by the operation panel. This signal includes not onlya character mode, a map mode, a character photograph mode, aphotographic paper photograph mode, a print photograph mode, etc. butalso a negative film mode and a positive film mode for an optional filmprojector, a mode (printer function) for inputting the image from anexternal apparatus, etc. Herein, the general character photograph modeis described.

First, a structure of the character edge reproducer 451 is described.Data Di₇₋₀ (C, M, Y and K₃₇₋₃₀) and maximum color data MAX₇₋₀ areinputted to delay memories 802 and 803 in order that they aresynchronized to the region discrimination results. The data Di₇₋₀ havebeen obtained by converting/correcting the R, G and B data to the C, M,Y and K data in the color corrector, and the data MAX₇₋₀ have beenobtained by logarithm correction on the R, G and B data. A selector 804selects Di₇₋₀ or MAX₇₋₀ for each color. MAX₇₋₀ is the signal selected instead of the normal K image data in a region which has been subjected tothe black character discrimination by the black character correctiondata. The output Di₇₋₀ of the selector 804 is inputted to four linememories 805 to 808, which are dependently connected, in order to expanda matrix of 5 lines*5 dots. The data (Dj, Dk, Dl, Dm and Dn₇₋₀) of fivelines from the line memories are inputted to a Laplacian filter 809, aMin filter 810 for the 5 dots*5 lines and a sharpness adjuster 811. Apredetermined sharpness adjustment image is selected by a selector 812in accordance with the status of a sharpness setting signal SD₂₋₀depending on the sharpness level set by the operation panel. Do₇₋₀ isoutputted for each of the C, M, Y and K data.

The Min filter 810 selects the data of the minimum gradation level fromthe data expanded in the 5*5 two-dimensional matrix, and it outputsDq₇₋₀. This is used for the removal of the outside data of the characteredge in order to remove the color components (C, M and Y) and to improvethe contrast during the black character discrimination. The Laplacianfilter 809 (shown in detail in FIG. 23) is a spatial filter for edgeemphasis using a 5*5 matrix. The filter results of the colors are onceinputted to a Laplacian table 813 in order to convert the data tooptimum data as the edge emphasis quantity. The data is then outputtedas DEDG₁₇₋₁₀. A selector 814 selects the edge emphasis signal DEDG₁₇₋₁₀obtained from the Laplacian filter of each color or the black edgecorrection signal VEDG₇₋₀ from the region discriminator. Then, theselector 814 outputs USM₁₇₋₁₀. The difference between DEDG₁₇₋₁₀ andVEDG₇₋₀, is as follows. The former is the edge correction signal for thecolor gradation level components (C, M, Y and K), while the latter isthe value edge correction signal obtained from the R, G and B data bythe secondary differential filter.

Then, a selector 815 selects whether or not the edge is emphasized. Theselector 815 outputs the final edge correction signal USM₂₇₋₂₀. On theother hand, the output data Do₇₋₀ of the selector 812 is inputted to aselector 816 and a smoothing filter (shown in detail in FIG. 24). Thedata Do₇₋₀ is selected together with the smoothing filter result Dp₇₋₀by the selector 816. Then, Dr₇₋₀ is outputted. The result Dq₇₋₀ or Dr₇₋₀of the 5*5 Min filter is selected by a selector 817 and then outputtedas Ds₇₋₀ to an adder for the edge emphasis. Finally, an adder 818 addsthe edge correction data USM₂₇₋₂₀ of each color to Ds₇₋₀ which the colorimage data are directly corrected into. Then, Dt₇₋₀ (C, M, Y and K₄₇₋₄₀)are outputted.

Accordingly, selection signals MPX4 to MPX0 for controlling thecharacter edge reproducer perform following control.

-   MPX0 (the selection of black character correction data): If MPX0 is    “L”, the first selector 804 selects MAX₇₋₀ (or the maximum color    data after the logarithm correction on R, G and B data). If it is    “H”, the selector 804 selects Di₇₋₀ (C, M, Y and K₃₇₋₃₀, C, M, Y and    K data after the color correction).-   MPX1 (the selection of black edge correction quantity): If MPX is    “L”, the second selector 814 selects the black edge correction data    VEDGI₇₋₀ from the region discriminator. If it is “H”, the selector    814 selects the correction data DEDG₇₋₀ for the edge emphasis from    the Laplacian filter 809 of the input C, M, Y and K data.-   MPX2 (the permission of edge emphasis): If MPX2 is “L”, the third    selector 815 selects the inhibition of edge emphasis (the edge    correction quantity=0). If it is “H”, the selector 815 selects the    permission of edge emphasis.-   MPX3 (the selection of smoothing filter): If MPX3 is “L”, the fourth    selector 816 selects the smoothing filter result. If it is “H”, the    selector 816 selects that the result of the sharpness adjustment is    allowed to pass through.-   MPX4 (the selection of Min filtering): If MPX4 is “L”, the fifth    selector 817 selects the result of the MIN filter in 5 dots*5 lines.    If it is “H”, the selector 817 selects that the result of the    foregoing fourth selector 816 is allowed to pass through.

As described above, the data inputted to the character edge reproduceris selected in accordance with MPX0. It is selected in accordance withMPX1 and MPX2 whether the correction data for the edge emphasis for theinput data is selected or inhibited. The correction of the input dataitself is selected in accordance with MPX3 and MPX4. The contents of theprocessing in the character edge reproducer are therefore determined bythe conversion of five types of select signals MPX4 to MPX0 fordetermining the correction in accordance with the result of the regiondiscrimination in the character reproduction controller.

It is described below how the character edge reproducer 451 actuallycontrols the character edge reproduction.

In color character photograph mode (MODE₃₋₀=“2” and −MCAREA=“H”), adocument region is judged from the result of the region discriminationin the following way. In the data, −AMIOUT denotes dot discriminationsignal, −PAPA denotes black character discrimination signal, −EDGdenotes character edge discrimination signal, −INEDG denotescharacter/background discrimination signal.

-AMIOUT -PAPA -EDG -INEDG Document region “L” “H” — — Dot region “L” “L”— “L” Black character in dot image “H” “L” — “L” Black character “H” “H”“L” “L” Color character “H” — “L” “H” Outside of character edge “H” “H”“H” — Continuous gradation level portion

Then, the K data in the color character photograph mode is controlled byMPX4-MPX0 in the following manner.

Document region MPX0 MPX1 MPX2 MPX3 MPX4 -LIMOS Dot region “H” “H” “L”“L” “H” “H” Black character in dot image “L” “H” “H” “H” “H” “H” Blackcharacter “L” “L” “H” “H” “H” “L” Outside of character “H” “H” “L” “H”“H” “L” Continuous gradation level portion “H” “H” “L” “H” “H” “H”

Further, the C, M and Y data in the color character photograph mode arecontrolled by MPX4-MPX0 in the following manner.

Document region MPX0 MPX1 MPX2 MPX3 MPX4 -LIMOS Dot region “H” “H” “L”“L” “H” “H” Black character in dot “H” “H” “L” “H” “L” “H” Blackcharacter “H” “H” “L” “H” “L” “L” Outside of character “H” “H” “H” “H”“H” “L” Continuous gradation level portion “H” “H” “L” “H” “H” “H”

This means following facts.

-   (1) For a dot region, each input color data is smoothed and the edge    emphasis is not permitted.-   (2) For a black character in a dot image, an edge component is    removed from the C, M and Y components by the 5*5 Min filter. The K    component is replaced by MAX(DR, DG, DB).-   (3) For a black character, the edge component is removed from the C,    M and Y components by the 5*5 Min filter. The K component is    subjected to edge emphasis by the value component and replaced by    MAX(DR, DG, DB).-   (4) For a color character, the C, M and Y components are subjected    to edge emphasis by the Laplacian filter of each color. The input    data of the K component is allowed to pass through.-   (5) For an outside of a character, the edge component is removed by    the 5*5 Min filter.-   (6) For a continuous gradation level portion, the input data of each    color is allowed to pass through.

Next, it will be explained how the character edge reproducer 451corrects each document region judged in accordance with the result ofthe region discrimination.

First, processing on a dot region is described. In an area judged as adot region, moiré pattern is prevented by the smoothing. The causes ofmoiré pattern are generally classified into three types:

-   -   (1) Interaction between a sampling period (resolution) and a dot        period when the image is read by the CCD sensor.    -   (2) Interaction between the frequency characteristic of the        spatial filter such as a Laplacian filter for edge emphasis and        the dot period.    -   (3) Interaction between a gradation level reproduction period        and the dot period when the gradation levels are reproduced by        the printer.

The type (1) is little visually noticeable at the resolution level ofabout 400 dpi.

The type (2) differs depending on the size and directivity of the filterfor edge emphasis. However, it can be solved by inhibiting the edgeemphasis in dot areas in a document. Thus, the edge emphasis isinhibited in the dot regions.

The type (3) depends on the pulse width modulation cycle in the printhead controller for determining the gradation level reproduction period.In the dot areas, moiré patterns are liable to occur due to the 2-dotpulse width modulation as described below. Thus, as shown in FIG. 24,three-dots in the main scan direction are subjected to integration typesmoothing in order to previously attenuate high frequency components ofthe dot frequency. Thereby, the interference with the gradation levelreproduction period is avoided.

Next, processing on a black character region in a dot image isdescribed. At present, it is not perfectly possible to distinguish atype of a document having black characters printed on a light colorbackground with a dot-like pattern from another type of a documenthaving black dots printed thereon, because detection of an isolated dotfor detecting dots coexists with character edge detection. Consequently,an intermediate processing is applied to an area in which the blackcharacter discrimination and the dot detection coexist. In the area, thesmoothing is not performed so that a black character is prevented frombeing blurred. The edge emphasis is not performed so that moiré patternsare prevented. In order to prevent color blur of a black character, theblack component is replaced by the maximum color data obtained from theR, G and B data after logarithm correction, and the edge component ofthe color components (C, M, Y) is attenuated with a minimum filter.

Next, black character discrimination in the character edge reproducer451 is described. In the black character discrimination, the C, M and Ycomponents are attenuated and removed by the Min filter 810 in order tocorrect the blurred color in an edge. The components are removed by theMin filter 810, thereby preventing the phenomenon caused by the too muchreduction of the components, i.e., preventing partial whitening of theperiphery of the character. The K component is replaced by the maximumcolor in R, G and B after the logarithm correction. The edge is enhancedin accordance with the value edge correction signal obtained from theminimum color of R, G and B. Thus, the data can be corrected into theclear black data which is resistant to copy generation. The clear blackcharacters are thus reproduced on a copy as if they were reproduced bythe use of black color alone.

It is explained here why the value edge correction signal obtained fromthe data MIN(R, G, B) is used as the edge emphasis quantity. Asmentioned above on the region discriminator, the value edge is sensitiveto the background (white background). On the other hand, it isinsensitive to gradation change at high densities and hard to generateimage noises. The value edge itself has characteristics to improvecontrast and to prevent narrowing a line, compared to the gradationlevel image data after logarithm correction. Both are affected by thelogarithm correction, and the influence of the logarithm correction on aline read can easily be seen in FIG. 25. In order to improve thegeneration of the character image, it seems better that contrast ofcharacters is enhanced relative to the white background and that theedge is enhanced a little over. Therefore, the edge is enhanced by theedge correction of the value component. In this case, because MIN(R, G,B) is used as the value component, the thickened image distribution isobtained when a line is read.

Next, it is described why the K component is replaced by the dataMAX(DR, DG, DB) before the edge emphasis. The gradation level of the Kcomponent is determined by the black paint processor in the colorcorrector. This value is 100% of the black paint quantity BP at maximum,i.e., MIN(DR, DG, DB). Accordingly, the K data after the colorcorrection has the following relationships: MAX(DR, DG, DB)>=MIN(DR, DG,DB)>=K data. Therefore, there is a tendency that MAX(DR, DG, DB) havingthe higher gradation level is suitable for the character reproductionthan the normal K data. More particularly, as shown in FIG. 26, this ismore evident for the reproduction of a thin line. The reason is thatthere are differences in the resolutions among R, G and B due to thecharacteristic of the lens for forming the image on the CCD sensor.Thus, when a black thin line is read, only data of low contrast isobtained as MIN(DR, DG, DB) due to the resolution difference. Thus, ablack thin line is unclearly reproduced and, as a result, it lacksclearness. Since the normal K data has the extremely low gradationlevel, the improvement of the contrast by the edge emphasis is limited.The K data is therefore replaced by MAX(DR, DG, DB) not affected by thisinfluence, whereby the reproducibility of a black thin line is improvedmuch, thus realizing the black character reproduction that does notdepend on the line width. FIG. 27 shows correction of blurred color inthe black character discrimination.

Next, it is described how a color character region is processed. Aregion which is not a dot region, not a black-character region and anin-character edge region are regarded as a color character region, andin the region, the C, M and Y color components are subjected to the edgeemphasis. In this case, the edge correction data for the edge emphasisis processed in accordance with the result of a Laplacian filter of eachcolor so as to prevent the color change at the edge due to the edgeemphasis. The as-received data of the K component is allowed to passthrough.

First, a processing at an outside region of a character edge isdescribed. At the side of the background (outside edge) decided by thediscrimination between the character and the background in a characteredge, a 5-lines*5-dots Min filter is used in order to achieve an unsharpmask effect (to increase the gradation level change at the edge) forimproving the contrast of the character reproduction and the edgeemphasis at the inside of the character. Because the minimum gradationlevel in the peripheral pixels is selected in the periphery of the edge,the gradation level is not extremely reduced in the periphery of theedge due to the substitution in accordance with the result of the Minfilter of each color. Therefore, typically, the edge emphasis by theLaplacian filter can prevent the periphery of a character from turningwhite.

Next, it is described how the continuous gradation level region isprocessed. A pixel which does not belong to any of the five types ofdocument regions described above is judged as a continuous gradationlevel portion, wherein without a particular correction, the as-receiveddata of each color is allowed to pass through.

Next, processing of the gradation reproduction attribute signal −LIMOSis described. The gradation reproduction attribute signal is transmittedtogether with the image data of C, M, Y and K for the purpose ofautomatically switching the gradation level reproduction to a followingprint imaging controller and the gradation reproduction cycle in theprint head controller. This signal is “L” level in a non-dot region(−AMIOUT=“H”), in a character edge region (−EDG=“L”) and in an insideedge region (−INEDG=“L”). The signal gives an instruction to perform thegradation level reproduction which gives priority to the resolution anddoes not distort the character. The pseudo 256-gradation-levelprocessing called the multi-value error diffusion is typically performedfor the gradation level reproduction in the print imaging controller. Inthe character edge corresponding to −LIMOS=“L”, the simple quantizationis, however, performed thereby preventing the edge from being distorted.

The 2-dot pulse width modulation reproduction whose screen angle is setto 45′ is typically performed in the print head controller. However, the1-dot pulse width modulation reproduction giving priority to theresolution is performed in a region corresponding to −LIMOS=“L”. Theprocessing is switched for an inside edge in the character edge, wherebythe gradation reproduction cycle of the print head controller isswitched at a character edge boundary. Thus, a gradation level jump dueto difference in the gamma-characteristic becomes harder to benoticeable.

In this manner, the C, M, Y and K data (C, M, Y and K₄₇₋₄₀) after theoptimum image correction in accordance with the result of the regiondiscrimination in the character edge reproducer are subjected to theimage adjustment in a color balance/gamma adjuster 452 in accordancewith the setting from the operation panel. Then, they are transmittedtogether with the −LIMOS signal to the print imaging controller.Thereafter, the data are subjected to the exposure control for formingan image on each color photoconductor by means of the light modulationusing the semiconductor laser in the print head controller.

Next, a gradation reproducer 500 of the print imaging controller isdescribed. The 8-bit data, which are obtained by converting the R, G andB data that are read by the image reader 200 into the C, M, Y and K databy means of the image processing, are simultaneously inputted to thegradation reproducer 500. The gradation reproducer 500 receives the8-bit image data of C, M, Y and K and the gradation reproductionattribute signal −LIMOS and converts them to the pseudo256-gradation-levels by the character separation type multi-value errordiffusion. The gradation reproducer 500 outputs each 3-bit color data(gradation level data) and 1 bit gradation reproduction attributesignal.

FIG. 28 is a block diagram of the gradation reproducer 500. Selectors901 and 902 select a test data for the resist detection or image datafrom the image reader 200. The selected 8-bit data ED₁₇₋₁₀ is convertedto 8 gradation level data obtained simply by substantially equallydividing 0-255 gradation level range into 7 by a 3-bit encoder 903 (seeFIG. 29). That is, the following encoding is performed.

Input gradation level data Encoded data   0-17 0 18-53 1 54-90 2  91-1273 128-164 4 165-200 5 201-238 6 239-255 7

On the other hand, an adder 904 adds ED₁₇₋₁₀ to feedback error dataED₄₇₋₄₀ subjected to error diffusion and outputs ED₂₇₋₂₀. Then, asubtractor 905 subtracts an offset quantity (OFFSET₇₋₀=18) from the dataED₂₇₋₂₀ obtained by the addition in order to cancel offset error dataprovided so as not to result a negative error by an error detectiontable 906, as described below. The contents of the error detection table906 are as follows. If Din−18≧239, Dout=(Din−18)−255+18. If238≧Din−18≧202, Dout=(Din−18)−220+18. If 201−Din−18≧162,Dout=(Din−18)−183+18. If 164≧Din−18≧128, Dout=(Din−18)−146+18. If 127Din−18≧91, Dout=(Din−18)−109+18. If 90≧Din−18≧54, Dout=(Din−18)−72+18.If 53≧Din−18≧17, Dout=(Din−18)−35+18. If 16≧Din−18, Dout=(Din−18)+18.Similarly to the value ED₅₇₋₅₀ obtained by the subtraction, encoding to3-bit data is performed by a 3-bit encoder 904 for encoding into 8gradation level data. A selector 908 selects the image data ED₇₂₋₇₀after the error diffusion or the image data ED₆₂₋₆₀ obtained by simplyencoding the input image data into 8 gradation level data, in accordancewith the gradation reproduction attribute signal.

In synchronization with the image data, the transmitted gradationreproduction attribute signal −LIMOS indicates the character edge if itis “L” level or indicates the continuous gradation level portion(non-edge) if it is “H” level. That is, a character edge is simplyencoded into a 3-bit data of 8 gradation levels, while a non-edge isencoded into a 3-bit data with error diffusion. Thus, a character edgedoes not have the distortion or texture unique to the error diffusion,while a continuous gradation level portion obtains the smooth gradationlevel reproduction due to the multi-value error diffusion. The 3-bit C,M, Y and K gradation level data after the the gradation levelreproduction are transmitted to a following drawing position correctortogether with the gradation reproduction attribute signal (3-bit data ofeach color).

Next, an error feedback route for the error diffusion is described. Asum value ED₂₇₋₂₀ of the feedback error ED₄₇₋₄₀ and the input image dataED₁₇₋₁₀ is inputted to the error detection table 906 in order todetermine the error data to be added for the subsequent pixel. First,the error detection table 906 subtracts the offset error quantity (=18)from the value. Then, the table 906 determines the gradation level errorin the gradation level range coinciding with a threshold level (=1, 7,53, 90, 127, 164, 201, 236) in the 3-bit encoder. Finally, the table 906adds the offset value (=18) equivalent to the maximum minus error valueso that the error can be integrated with weights at high speed in anerror diffusion matrix 911. A series of processing is calculated bymeans of a lookup table, to output error data ED₃₇₋₃₀. The contents ofthe table are downloaded by a central processing unit in the printimaging controller. The contents can be changed in connection with thethreshold level of the 3-bit encoding and the gradation level of agradation level decoder described below. In this embodiment, the errordiffusion is accomplished by equally dividing the 0-255 gradation levelrange into seven. However, the following processing can be alsoperformed. For example, if priority is given to the highlight gradationlevel, the threshold levels in the 3-bit encoding are set to values moretowards to zero. In accordance with this setting, the gradation level ofthe gradation level decoder and the gradation level difference in theerror detection table are set by the central processing unit in theprint imaging controller to download them. Then, this can realize veryflexible gradation level reproduction. This approach permits calculationof a series of processing in the table at high speed.

The output error data ED₃₇₋₃₀ are subjected to integration with errorweights near an objective pixel by the error diffusion matrix 911 by theuse of line memories 909 and 910. The feedback error data ED₄₇₋₄₀ of thesubsequent image data is outputted. At an stage in which the data isoutputted from the error detection table 906, the error data is offsetso that the maximum minus error value (=−18) may be canceled and theerror value may be set to zero. Thus, the error diffusion matrix doesnot need an operation on negative values (or only simple adder circuitsare needed). Consequently, the circuit operation is at high speed andits scale can be small.

An error feedback system must be operated at high speed for thefollowing reason. When a transmission rate of the input C, M, Y, K imagedata is fast, it is necessary to perform error diffusion operationbefore the subsequent pixel data enters to the system.

The drawing position controller 510 in the print imaging controller hasthe following two functions.

-   (1) The image is stored in a memory for the time delays caused due    to the positions of the photoconductors located along the scan    direction. The image is delayed and outputted.-   (2) In the control of the main scan position, a start position of    the drawing in the main scan direction is controlled, in order to    correct an error of a position in the main scan direction of the    print head, and the mirror image phenomenon of the C and M data due    to the structure of the print head is corrected.

FIGS. 30A and 30B show the drawing position controller 510 for thesubscan. Although the same circuits are provided for the four colors C,M, Y and K, these circuits differ in the number of subscan delay controlDRAM modules 513. First, a data selector 511 selects either the data C,M, Y and K₂₃₋₂₀ transmitted from a gradation reproducer 500 or the dataC, M, Y and K₂₃₋₂₀ transmitted from the frame memory 520 according to anFSEL signal set by a reference position signal generator 550. The 4-bitimage data of 8 dots in the main scan, selected by the data selector 511are inputted as one pack of serial data to an 8-dot S/P converter 512.The converter 512 coverts the serial data to 32-bit parallel data. Inthe following DRAM control, the data is thus read from and written tothe memory by using eight dots as one cycle.

The subscan delay control DRAM modules 513 (shown in detail in FIG. 30C)perform delay control on the C, M, Y and K data in the subscandirection. The memory is controlled by addresses ADR₉₋₀, RAS, −CAS₀, ₁,₂, WE and −OE which are outputted from a DRAM controller 514. The delayquantity of the subscan is determined in accordance with the differencein the count values between a write address counter and a read addresscounter. That is, the initial value of the write address counter is “0”,while the initial value of the read counter is VSA₁₁₋₀ which is set bythe central processing unit in the print imaging controller. Thus, thedelay quantity of each color corresponds to the VSA₁₁₋₀ lines. The readand write address counters generate the addresses in the main scan andsubscan directions. The main scan address is counted by VCLK (imagesynchronization clock) and reset to the initial value by −TG (main scansynchronization signal). The subscan address is counted with the −TGsignal. For the read address, the count value is periodically loaded toVSA₁₁₋₀ set by the central processing unit in the print imagingcontroller, as described above. For the write address, the count valueis loaded to zero. The address to the DRAM modules 513 is selected fromthese count values by the following address selector in synchronizationwith the DRAM control operation.

−FREEZE is a signal transmitted from the reference signal generator 550.This signal repeats “L”/“H” line by line when an image is copied on anoverhead projector paper or a thick paper. (It is “H” during the normalcopy.) For reproducing the image on an overhead projector or a thickpaper, it is necessary to reduce a paper feeding speed to ½ of the speedfor the normal copy because of thermal conductivity of the fixing unit.In this case, the subscan is operated so that the image is reproduced at800 dpi. However, the normal 800-dpi operation needs delay memorieshaving a double capacity for each color. The corrector for the subscandistortion also needs first-in first-out buffer memories having a doublecapacity. In the case of the data of 800 dpi, a double amount of tonersadhere to the paper. Thus, it is necessary to insert white data for eachline. During the control at half a speed, the following operation istherefore performed in order to inhibit the read/write operation of thissubscan delay memories for each line. That is, if −FREEZE=“L”, a DRAMcontrol signal outputted from a control pulse generator in the DRAMcontroller 514 is becomes inactive. Also, the read and write addresscounters are stopped so as to prevent the counters from counting theaddress. Thereby, it is not needed to increase the memory capacity.

Next, an 8-dot P/S converter 515 converts the 32-bit parallel image datafor 8 dots outputted from the subscan delay control DRAM modules 513into the original serial data C, M, Y and K₄₃₋₄₀. Then, the converter515 outputs the serial data.

FIG. 31 shows a main scan drawing position corrector 516. The data C, M,Y and K₄₃₄₀ transmitted from the subscan drawing position controller isinputted to the main scan drawing position corrector 516. After the mainscan drawing position correction and the mirror image processing of thenecessary data, the data C, M, Y and K₅₃₋₅₀ is outputted to the imagedistortion corrector 540. A main scan drawing position memory 5161comprises two memories, connected in parallel, which can store the dataof one line in the main scan. The write and read operations arealternately switched for the memory by a line toggle counter 5162.

For both the write and read addresses for the main scan drawing positionmemory 5161, the image synchronization clock VCLK is counted by counters5163 and 5164, thereby generating a main scan address. In order to setthe address counters 5163 and 5164 to an initial value at the start ofthe main scan, the following operation is performed. The main scansynchronous signal (−TG) is reset or inputted as a load signal. Thewrite count value is reset to “0”, and the read count value is loaded toHSA₁₂₋₀ set by the central processing unit in the print imagingcontroller. Because the laser scan raster direction of the C and M datais opposite to that of the reference color signal K data, the writeaddress counters 5163 is allowed to count down from the initial value“0”.

Thus, an normal image control is performed by UDSEL for the Y and Ksignals=“H”. A mirror image control is performed by UDSEL for the C andM signals=“L”. HSA₁₂₋₀ set to the read address as the load value by thecentral processing unit in the print imaging controller, indicates aposition at which the drawing is started in the main scan direction.Thus, this value permits controlling the main scan drawing startposition for each color. The K image data sets drawing positions for themain scan and for the subscan so that the image is drawn on appropriatedrawing positions on a paper fed onto the transfer belt 304. The otherdata C, M and Y set the drawing position relative to the K image data.

FIGS. 32A and 32B show the image distortion corrector 540 which correctsthe image distortion in the main scan and subscan directions for the4-bit data C, M, Y and K₅₃₋₅₀ transmitted from the drawing positioncontroller 510. The corrector 540 outputs 9-bit data C, M, Y and K₇₈₋₇₀to the print head controller. The image distortion corrector 540 has thefollowing two functions.

-   (1) A memory device stores data as to a number of lines    corresponding to the maximum width of the distortion (bow and skew    distortions) of the image on the transfer belt 304 in the subscan    direction, the distortions being caused due to the relative shift of    the laser exposure position on each color photoconductor. The    distortion along the subscan direction is interpolated and    outputted.-   (2) A flip-flop circuit stores the data as to the number of dots    corresponding to the maximum width of the distortion (main scan    magnification distortion) of the image on the transfer belt 304 in    the main scan direction, the distortion being caused due to the    relative shift of the laser exposure position on each color    photoconductor. The distortion along the main scan direction is    interpolated and outputted.

The black data is used as the reference of the above-described imagedistortion correction. In order to correct the relative distortion ofthe other three colors C, M and Y, the image distortion correction isnot performed for the black data K₅₃₋₅₀. Meanwhile, for the other dataC, M and Y₅₃₋₅₀ the correction data is generated and the distortion isinterpolated for each color so that the C, M and Y correspond with thedistortion of the black data. The same circuits are provided for threecolors C, M and Y.

As shown in FIGS. 33A and 33B, in the subscan image distortioncorrection, the data is first transmitted to a first-in first-out (FIFO)buffer memory 541 capable of storing the data corresponding to themaximum distortion width (24 lines). The FIFO buffer memory 541 stores24 lines of the image data C, M, Y and K₅₃₋₅₀ continuously transmittedfor each line. A read/write clock of the FIFO buffer memory 541 is VCLK.The address is reset by the −TG signal. The FIFO buffer memory 541comprises line memories connected dependently, so that the data issequentially delayed line by line. If −FREEZE=“H”, the operation isperformed in the same manner as the operation for stopping theread/write of the subscan delay control DRAM. That is, the −FREEZEsignal makes RE/−WE signal enable, and the operation is stopped for eachline, thereby performing the half speed control for the 800-dpioperation.

The delay data of each FIFO buffer memory is inputted in parallel to animage selector 542. In order that the following density distributor iseasily operated, the data of two adjacent lines is outputted in parallelfrom data of 24 lines*4 bits supplied from the FIFO buffer memory 541 inaccordance with a select control terminal S₄₋₀. That is, when Xout₃₋₀selects n-th line delay data, Yout₃₋₀ outputs (n+1)-th line delay data.The signal to be selectively outputted to Xout₃₋₀ is selected fromXin00₃₋₀ to Xin23₃₋₀. The signal is determined by a 5-bit signal of asubscan interpolation data KD₁₇₋₁₃.

A gradation level decoder 543 (shown in detail in FIG. 34) converts ordecodes bits 2-0 of Din₃₋₀ of input gradation level code to thegradation level corresponding to the threshold level for the 3-bitencoder in the gradation reproducer. That is, the code is changed in thefollowing manner.

Input code (Din²⁻⁰) Gradation level (Dout⁷⁻⁰) 0 → 0 1 → 35 2 → 72 3 →109 4 → 146 5 → 183 6 → 220 7 → 255Dout₈ represents the gradation reproduction attribute signal of eachcolor, and it is passed through as Dout₃.

If −FREEZE signal=“L”, the data is replaced by white (“00”) for eachline so that the amount of toner adhesion is equivalent to that of toneradhesion for the normal 400-dpi operation.

A density distributor 544 performs interpolation of density distributionfor each ⅛ dot by using the data of two adjacent lines. That is, byassuming that A is n-th line gradation level data and B is (n+1)-th linegradation level data, the interpolation is as follows.

KD¹²⁻¹⁰ = 0 → Y = A KD¹²⁻¹⁰ = 1 → Y = (7A + B)/8 KD¹²⁻¹⁰ = 2 → Y = (3A +B)/4 KD¹²⁻¹⁰ = 3 → Y = (5A + 3B)/8 KD¹²⁻¹⁰ = 4 → Y = (A + B)/2 KD¹²⁻¹⁰ =5 → Y = (3A + 5B)/8 KD¹²⁻¹⁰ = 6 → Y = (A + 3B)/4 KD¹²⁻¹⁰ = 7 → Y = (A +7B)/8

A mixture ratio of inputs A:B with respect to output Y is changed inaccordance with the subscan interpolation data. By assuming that acorrection quantity due to the distortion is q lines, the interpolationdata KD₁₇₋₁₀ is 8*q. Thus, the distortion corrector 540 can correct thedistortion with high accuracy at every ⅛ dot within the width of 24lines. That is, the gradation reproducer 500 reduces a memory capacityof the delay memory needed for the subscan drawing position control to ½(this is similar for an FIFO buffer memory in the image distortioncorrector) by encoding the data into the 4-bit data, while maintainingthe 8-bit image quality. In an interpolator which does not need a largememory capacity, the gradation level is decoded to the 8-bit data sothat the interpolation can be performed with high accuracy, therebydistributing the density. FIG. 7 shows an example of the imagedistortion correction using the density distribution for the subscan.

After the density distribution for the subscan, the data is outputted asC, M and Y₆₇₋₆₀ to the image distortion corrector for the main scan.

On the other hand, bit 8 indicating the gradation reproduction attributein the density distributor is processed by using the data of twoadjacent lines similarly. Now by assuming that the attribute signal ofn-th line is A and the attribute signal of (n+1)-th line is B, theprocessing is as follows.

KD¹²⁻¹⁰ = 0 → Y = A KD¹²⁻¹⁰ = 1 → Y = A KD¹²⁻¹⁰ = 2 → Y = A KD¹²⁻¹⁰ = 3→ Y = An or B (edge if A or B is an edge) KD¹²⁻¹⁰ = 4 → Y = A or BKD¹²⁻¹⁰ = 5 → Y = A or B KD¹²⁻¹⁰ = 6 → Y = B KD¹²⁻¹⁰ = 7 → Y = BWhen the amount of shift of the line from the reference position (Kdata) is small (within ± 2/8 line), the edge attribute data of a nearbyline is used. When the amount of shift of the line from the referenceposition is large (±⅜ or ± 4/8 line), a reference is made to an OR ofboth the edge information (priority is given to the edge). The data,which is selected and judged as the edge, is then outputted as C, M andY ₆₈ to the main scan image distortion corrector.

As shown in FIGS. 35A and 35B, a main scan image distortion corrector516 performs interpolation as in the case of the subscan distortioncorrection. Differently from the subscan, a shift register 5171 usingflip-flop circuits is used in order to generate continuous delay dataalong the main scan direction in stead of the FIFO buffer memory. Inthis case, the width of the maximum distortion correction is constitutedso that 9-bit data can be delayed by 32 dots. An image selector 5172selects the data of two adjacent dots in parallel. Since the value isalready decoded to gradation level, a decoder is not needed. A densitydistributor 5173 processes the data between two adjacent dots. Thedensity distribution and the selection of the image of two adjacentlines are accomplished in accordance with the main scan interpolationdata KD₂₇₋₂₀.

An image distortion correction coefficient data generator 548 shown inFIG. 35B generates a correction data for correcting the image distortionalong the main scan and subscan directions by using a main scan addresscounter 5481 and two types of lines memories 5482 and 5483 for thecorrection. The data on the amount of correction of the image distortionalong the main scan and subscan directions to be corrected iscontinuously changed depending on the main scan position (address).Accordingly, the central processing unit of the print imaging controllerexpands continuous correction distribution data of one line inaccordance with the amount of shift of the C, M and Y images from the Kimage obtained by the resist sensor. Then, the amount of correction iscreated for each main scan pixel.

As described above, the K image is the reference image data for the C, Mand Y images. In order to form an image on the transfer belt 304 on aproper position on the paper, the vertical and main scan positions ofthe K data are determined by the delay memory devices and by the mainscan drawing position controller, respectively, in the drawing positioncontroller 510. However, three resist sensors along the main scandirection are not arranged at the proper positions on the transfer belt304 along the main scan direction, i.e., the sensors are not arranged atthe same positions for each machine. Therefore, the relativerelationship is not fixed between the sensor detection position and theaddress on two types of line memories (main scan and subscan imagedistortion correction RAMs 5482 and 5483) for expanding the correctioncoefficient. Thus, it is necessary to shift the distribution of thecorrection data in accordance with the sensor position obtained from theK resist image. The central processing unit in the print imagingcontroller changes the data expanded in the memory as to the distortioncorrection quantity in accordance with the sensor detection position.

A position at which a main scan address counter 3581 starts reading canbe changed by ADRSET₁₂₋₀ (common to C, M and Y) set by the centralprocessing unit in the print imaging controller. This counter countsVCLK, and the count value is loaded to ADRSET₁₂₋₀ by the −TG signal. Thechange in this value is controlled in the following reason.

When data is transmitted from the print image controller to the printhead controller, an image of the image reader 200 always has a referenceposition at one side because a document is put the platen glass platewith reference to the end of the platen glass plate for the main scan.However, the image of the image forming section 300 has a reference atthe center so that a paper is fed on the basis of the center position ofa polygon motor (or the center of the transfer belt). Thus, as shown inFIG. 37, an interface between the print image controller and the printhead controller comprises an interface FIFO memory. The image outputtedfrom the print image controller is transmitted to the print headcontroller by converting the one-side-based image into the center-basedimage. FIG. 38 shows the timing chart for this processing. The main scanreference signal −TG of the image reader is used as an address writereset −WRES for the interface, and a main scan enable region signal −HDis used as a write enable (−WE), thereby controlling the write to theinterface FIFO memory device. Signal −SOS is a scan start signal for thelaser diode which is generated for each line according to the rotationof the polygon, and −HIA is a main scan drawing area signal used as aread enable signal −RE so as to control the read from the interface FIFOmemory. Since both of −TG and −SOS are the main scan reference signals,they have the same cycle. Signal −TG is used as the reference of theread of the CCD sensor, and signal −SOS is used as the reference of thewrite of the laser scan. The signal −HD can be changed on the basis ofthe signal −TG in accordance with the range of from one side to theimage read area. The signal −HIA can be changed on the basis of thecenter position of the signal −SOS in accordance with the width of themain scan of a paper to be fed.

An address of the memory device for expanding the image distortioncorrection data is generated on the basis of signal −TG. For the datageneration, it is necessary to change the position for expanding thecorrection data, in accordance with the size of a paper to be fed,because the data is guided by detecting the resist pattern on thetransfer belt 304, as described below. However, much time is wastedbecause the distortion correction data is expanded in the correctionmemory after the paper size is determined. Therefore, the load value ofthe main scan address generator is changed in accordance with −HIAchanged with the paper and the start position from the −SOS signal,whereby the main scan position of the distortion correction coefficientis adjusted.

After the correction of the image distortion, the K data is selected asdata (1FF(h)) of uniform black in the interface with the print headcontroller before it is written to the FIFO memory. This means that thebill recognizer 415 in the image processor of the image reader judgeswhether or not the document on the platen glass plate is a bill. If thedocument is judged as a bill, the whole surface of the image is paintedwith black data so as to avoid a normal copy. In a conventional fullcolor copying machine in which an image is scanned four times, a bill isrecognized when the C, M and Y images are formed before the black imageis formed, and the image is painted in black when the K image is formed.On the other hand, the color copying machine like the present system forsimultaneously copying four colors by one scan needs to judge whether ornot a document should be painted in black while scanning the image whilescan is performed. However, it is necessary for the bill recognition toselect some extent of the document region any time and to performpattern matching with a certain reference pattern in order to judgewhether or not the document is a bill. Thus, a some time is required forthe determination of the image read position during the scan. Sometimes,the image read position is not yet determined at the time of the imageforming (or the bill recognizer 415 judges the document as a bill afterthe image of the bill is formed on a paper). Thus, the painting iscontrolled after the subscan drawing position is controlled. Thereby,the K image is delayed and controlled for at least the timecorresponding to the distance between the photoconductors. A normal copyoperation can be inhibited if the bill recognition is only completed forthe time between the scan start and the forming of the K image on thephotoconductor.

The C, M, Y and K images, C, M, Y and K₇₈₋₇₀, which have been subjectedto the image distortion correction in the main scan and subscandirections, are transmitted to the interface (FIG. 37) between the printimaging controller and the print head controller. The drawing positionis shifted with reference to the paper. The data are transmitted to theprint head controller shown in FIG. 8. The data are optically modulatedand exposed on each color photoconductor, whereby an image is formed.

Next, it is described how the amount of shift from the resist sensor isfeed-backed. FIG. 39 shows a resist detecting pattern. The resistdetecting pattern is generated by the test data generator 530. Thispattern is selected as an image data by the gradation reproducer 500.Three Z-shaped data are generated along the main scan direction in eachof the C, M, Y and K resist patterns.

The print image controller controls the drawing under the followingconditions.

-   (1) As the amount of subscan delay C, M, Y, K_VSA₁₁₋₀ of each color    for controlling the subscan drawing position, are set to have the    same control value for all the C, M, Y and K data.-   (2) As the main scan drawing start position, C, M, Y, K_HSA₁₂₋₀, of    each color for controlling the main scan drawing position, all of C,    M, Y, BK have a control value so that the K image is drawn on the    proper position on the transfer belt.-   (3) The mirror image processing is performed for C and M in (2).-   (4) The image distortion correction values for both of main scan and    subscan are set to zero (all the data KD₁₇₋₁₀ and KD₂₇₋₂₀ are 0).

The color shift data transmitted from the resist sensors to the centralprocessing unit in the print imaging controller are the amounts (Vck₁₋₃,Hck₁₋₃, Vmk₁₋₃, Hmk₁₋₃, Vyk₁₋₃, Hyk₁₋₃) of main scan and subscan colorshift of each sensor relative to K, and the amount Tk₁₋₃ Of positionshift of each sensor calculated from the K image. Thus, the amountsVvk₁₋₃, Vmk₁₋₃, Vck₁₋₃ of color shift of C, M and Y relative to K aresubstantially the same as the distances between the photoconductors forthe colors.

The amounts Vyk, Vmk, Vck of subscan shift are calculated in accordancewith the difference in the time when each color Z pattern passes on thesensor for the first time. An oblique line of the Z-shaped pattern tiltsat 45°. Thus, the lateral (main scan) shift of the position can becalculated by knowing the time when a horizontal line and the obliqueline pass on the sensor. The amount Hck₁₋₃, Hmk₁₋₃, Hyk₁₋₃ of main scancolor shift of each color from K are determined from the differencebetween the amount Hk₁₋₃ of position shift of K and the amounts Hc₁₋₃,Hm₁₋₃, Hy₁₋₃ of position shift of the colors. The position α₁₋₃, whichthe each sensor is attached at, can be calculated from a predeterminedvalue β₁₋₃ of a print address of the Z pattern and the amount Hk₁₋₃ ofmain scan color shift of K. In the correction of the subscan direction,a delay control quantity VSA₁₂₋₀ of the subscan drawing position controlof each of C, M, Y and K is determined in the following manner. Assumingthat K_VSA₁₁₋₀ is Q1,Y_VSA¹¹ ⁻ ⁰ = Q1 − (Vyk₁ + Vyk₂ + Vyk₃)/3 − 12, M_VSA¹¹ ⁻ ⁰ = Q1 − (Vmk₁ + Vmk₂ + Vmk₃)/3 − 12  andC_VSA¹¹ ⁻ ⁰ = Q1 − (Vck₁ + Vck₂ + Vck₃)/3 − 12.

Next, in the subscan distortion correction memory, the amount of shiftof each color from the K image is expanded into a quadraticapproximation curve in the main scan direction shown in FIG. 40. In thedistortion corrector, the amount of shift correction equivalent to qlines is 8*q for the interpolation data KD₁₇₋₁₀. In this case, theposition of the resist sensor with respect to the main scan address iscorrected by the value Hk₁₋₃, and the correction data is expanded. Whenany correction data at any main scan position expanded in the memory isequal to or smaller than 0 (KD₁₇₋₁₀=0) or equal to or larger than 24(KD₁₇₋₁₀=191), the central processing unit in the print imagingcontroller clips the correction data at the upper and lower limitvalues.

In the correction of the main scan direction, a drawing start positionaddress HSA₁₂₋₀ of the main scan drawing position control of each of C,M, Y and K is determined in the following manner. Assuming thatK_HSA₁₁₋₀ is Q2,Y_HSA¹¹ ⁻ ⁰ = Q2 − 16 − (Hyk₁ + Hyk₂ + Hyk₃)/3, M_HSA¹¹ ⁻ ⁰ = Q2 − 16 − (Hmk₁ + Hmk₂ + Hmk₃)/3,  andC_HSA¹¹ ⁻ ⁰ = Q2 − 16 − (Hck₁ + Hck₂ + Hck₃)/3.

Next, in the main scan distortion correction memory, the amount of shiftof each color from the K image is expanded into a quadraticapproximation curve in the main scan direction shown in FIG. 41.

In this case, the position of the resist sensor with respect to the mainscan address is corrected by Hk₁₋₃. In the distortion corrector, theamount of shift correction equivalent to q dots is 8*q for theinterpolation data KD₂₇₋₂₀. At this time, the position of the resistsensor with respect to the main scan address is corrected by the valueHk₁₋₃, and the correction data is expanded. When any correction data atany main scan position expanded on the memory device is equal to orsmaller than 0 (KD₁₇₋₁₀=0) or equal to or larger than 32 (KD₂₇₋₂₀=255),the central processing unit in the print imaging controller clips thecorrection data at the upper and lower limit values.

FIGS. 42A and 42B show the frame memory 520. In the double-sided copyoperation of the system (for a A4/horizontal paper), an image is formedon five sheets of paper on the transfer belt and along the paperreversal path. A multi-double-sided copy operation is thereforeperformed by repeating the front-side copy and back-side copy every fivesheet. Thus, the image reader needs to temporarily store, in the framememory, the C, M, Y and K data on the document surface corresponding tothe front-side copy. A document surface corresponding to the back-sidecopy is repeatedly read by the image reader (in the same manner as anormal copy). These function of the memory and the controller forcontrolling this memory are performed by the frame memory 520.

In DRAM controller 4401, the main scan address is counted with VLCK(image synchronization clock). The address is cleared by the −TG signal(main scan synchronous signal), and the signals −RAS, −CAS and −WEneeded for DRAM control are generated. For the subscan, the address iscounted by the TG signal and cleared by −VD signal (subscan enableregion signal). At the same time, data writable area signals −C, M, Y,K_WE and data readable area signals −C, M, Y, K_RE are received. Thesignals WE and −CAS to a DRAM module 4402 are controlled so that theyare allowed or inhibited, whereby the write/read can be performed foreach region independently of each color. Particularly, the WE signal isenabled at a predetermined timing in the area in which any one of the−C, M, Y, K_WE signals is active (“L”). In this case, in the area inwhich the −C, M, Y, K_WE signal is active, −C, M, Y, K_CAS signal isallowed to be independently outputted, thereby controlling the write ofeach color to an optional area of the DRAM module. In the area in whichany one of the −C, M, Y, K_RE signals is active, the WE signal is notenabled, and by making −C, M, Y, K_CAS signal active, it is possible toread each color data from the DRAM module in a predetermined area. Thesignal −RAS is always outputted at a predetermined timing, thus ensuringthat the memory device is refreshed. The DRAM module comprises aplurality of DRAMs, and the DRAM module has an area for storing thecolor data C, M, Y, K on one surface of a A3-size document. The data iswritten and read in accordance with WE, −CAS and −RAS from the DRAMcontroller 4401.

The input/output of image data are processed in the same manner as thesubscan side of the drawing position controller. That is, for the input,one pack of 8 dots of the main scan is converted from serial to paralleldata, to write the 32-bit parallel data. For the output, conversely, thedata is converted from parallel to serial data, and the 4-bit serialdata is read. For the input, when −WHDWR signal is active (“L”), thedata (4h) for initializing the memory device is used as the input datato the frame memory, thereby erasing the memory device in accordancewith the write control. When −WHDWR signal is inactive (“H”), the dataC, M, Y and K₂₃₋₂₀ from the gradation reproducer 500 are used as aninput data to the frame memory, thereby each color data is controlled tobe written to the memory device. For the output, when −C, M, Y, K_CLRsignal is “H”, a predetermined value (4h) is used as the output datafrom the frame memory, thereby using the value as transmission data C,M, Y and K₃₃₋₃₀ to the following step (the drawing position controller).This is performed in the following reason. In an area where the 1-nodecontrol in an area (−HD=“H” or −VD=“H”) that is not an enable area onthe main scan and subscan sides and the color data readable area signals(C, M, Y, K_RE) are inactive, the image data is cleared and outputted.This memory control is used so as to print the sequential data C, M, Yand K transmitted from an external apparatus. This operation isperformed in the following manner. The C, M, Y and K image data issequentially received and sequentially written to a predetermined framememory for each color. The four colors are simultaneously read, and afull color print is performed.

Although the present invention has been fully described in connectionwith the preferred embodiments thereof with reference to theaccompanying drawings, it is to be noted that various changes andmodifications are apparent to those skilled in the art. Such changes andmodifications are to be understood as included within the scope of thepresent invention as defined by the appended claims unless they departtherefrom.

1. An image processor which converts image data of red, green and blueto image data of cyan, magenta, yellow and black, comprising: an areadiscriminator which discriminates a black character area in the imagedata of red, green and blue; a substitution processor which substitutesimage data having a maximum density in the image data of red, green andblue for image data of black belonging to the black character areadiscriminated by said area discriminator; and an edge emphasis processorwhich performs edge emphasis on the image data substituted for the imagedata of black by said substitution processor.
 2. The image processoraccording to claim 1, wherein said edge emphasis processor comprisestwo-dimensional differential filters in four directions.
 3. The imageprocessor according to claim 1, further comprising a corrector whichdeletes an edge component in image data of cyan, magenta and yellowbelonging to the black character area discriminated by said imagediscriminator.
 4. The image processor according to claim 3, wherein saidcorrector comprises a minimum filter which converts the image data of anobject pixel of each of cyan, magenta and yellow to image data having aminimum density in the object pixel and adjacent pixels thereto.
 5. Animage processing method of converting image data of red, green and blueto image data of cyan, magenta, yellow and black, comprising the stepsof: discriminating a black character area in the image data of red,green and blue; substituting image data having a maximum density in theimage data of red, green and blue for image data of black belonging tothe black character area discriminated in said discriminating step; andperforming edge emphasis on the image data substituted for the imagedata of black in said substituting step.
 6. The method according toclaim 5, wherein two-dimensional differential filters in four directionsare used in said step of performing edge emphasis.
 7. The methodaccording to claim 5, further comprising the step of deleting an edgecomponent in image data of cyan, magenta and yellow belonging to theblack character area discriminated in said discriminating step.
 8. Themethod according to claim 7, wherein a minimum filter is used in saiddeleting step which converts the image data of an object pixel of eachof cyan, magenta and yellow to image data having a minimum density inthe object pixel and adjacent pixels thereto.